1. Field of the Invention
The present invention relates to a polyhydroxyalkanoate (hereinafter abbreviated as “PHA”) containing a novel structural unit and a production process for the same. For instance, the present invention relates to a novel PHA that contains a monomer unit having a cyclohexyl structure at its side chain, and a production process for the PHA using an alkanoic acid as a raw material and also using a microorganism having an ability to produce and accumulate PHA in the microbial cell.
Furthermore, the present invention relates to a binder resin that can be used for an electrostatic charge image-developing toner, an electrostatic charge image-developing toner, and an image-forming method and an image-forming apparatus using the toner. For instance, the present invention relates to a binder resin, an electrostatic charge image-developing toner, an image-forming method, and an image-forming apparatus, which are used in electrophotography, electrostatic recording, and electrostatic printing by a copying machine, a printer, a facsimile machine, and so on where a toner image is formed on a latent-image bearing member (hereinafter, also simply referred to as an image bearing member) and is then transferred to a transferring material to form an image thereon. More concretely, the present invention relates to a binder resin having biodegradability so that the binder resin can be easily subjected to a waste treatment, and excellent fixing ability (low-temperature fixing ability, fixing temperature, and offset resistance) and blocking resistance, and also having a hydrolyzing ability for allowing a presently available deinking system to be directly used for the binder resin so that the binder resin can be easily deinked, and an electrostatic charge image-developing toner that contains the binder resin, and an image-forming method and an image-forming apparatus using the toner.
2. Related Background Art
Conventionally, many electrophotographic methods have been known in the art. Typically, each of the electrophotographic methods includes: forming an electric latent image on an image bearing member (a photosensitive body) by one of various kinds of means using a photoconductive material; developing the latent image with a toner to generate a visualized image; transferring a toner image to a transferring material such as a sheet of paper as needed; and fixing the toner image on the transferring material by means of, for example, heat and/or pressure to obtain a duplicate. As a method for visualizing the electric latent image, a cascade developing method, a magnetic blush developing method, a press developing method, and so on are known in the art. Furthermore, there is also known a method using a rotary developing sleeve having a magnetic toner placed thereon and having a magnetic pole arranged on the center of the developing sleeve to allow the magnetic toner to fly from the developing sleeve to a photosensitive body in the presence of a magnetic field.
As a developing system to be used for the development of an electrostatic latent image, there are two systems known in the art. The two systems are a two-component developing system using a two-component developer constructed of both a toner and a carrier and a mono-component developing system using a mono-component developer constructed only of a toner without the use of a carrier.
Here, a colored fine particle generally described as a toner includes a binder resin and a colorant as essential ingredients and includes a magnetic powder or the like as needed. By the way, the binder resin occupies most of the toner, so that physical properties of the toner can be extensively influenced by physical properties of the binder resin. For example, there is the need of providing the binder resin with delicate hardness and thermal melting characteristics. In addition, a toner obtained by pulverizing and classifying a binder resin where a coloring agent or the like is dispersed is requested to show good flowability without causing fine powders as a result of mechanical impacts by agitation in a developing device and without making the toner to be agglutinated. Furthermore, at the time of fixing, the toner should be promptly melted at low temperatures, and also at the time of melting, the melted toner should be of agglutinative. In other words, the physical properties of the toner can be controlled by controlling the physical properties of the binder resin.
Conventionally, as the binder resin, a styrene/acrylic ester copolymer, a polyester resin, an epoxy resin, an olefin resin, or the like has been used. Among them, the polyester resin is widely used as a toner resin for heat-roll fixation mainly because the toner has advantages in that the toner shows good dispersibility to a toner additive such as carbon black and good wettability to transfer paper when the resin is melted and also shows an excellent fixing ability.
In recent years, from the viewpoint of environmental protection, recycling of resources, a reduction in waste, an improvement in safety of waste, and so on have been considered on a worldwide basis. Those challenges are also concerned in the field of electrophotography without exception. That is, with the wide spread of copying machines and printers, the quantity of disposals including toners fixed on paper, waste toners after use, and printed sheets of paper is increasing every year. Typically, the conventional toner is hardly degradable because the toner is constructed of structural components respectively made of stable artificial compounds. Thus, the toner may remain in various environments including soil and water for a long period. Furthermore, for the recycling of resources, one of the important tasks is to recycle and reutilize plain paper. However, the conventional binder resins, notably styrene-based resins, are hardly deinked with alkali hydrolysis which is a problem in recycling plain paper. In addition, from the standpoint of maintenance of the earth environment or an influence on the human body, the safety of waste is also an important problem.
Under such a circumstance, persons skilled in the art have pursued the development of resins harmless to the human body and biodegradable with the action of microorganisms, i.e., biodegradable resins. There is described that many microorganisms produce biodegradable resins having polyester structures (hereinafter, referred to as polyhydroxyalkanoates, abbreviated as “PHA's”) and accumulate the resins in the microbial bodies, respectively. It is known that such PHA's may have various kinds of compositions and structures depending on the species of microorganisms to be used for the production, the compositions of media, the conditions for culturing these microorganisms, and so on. Heretofore, from the viewpoint of improving the physical properties, studies have been mainly conducted on regulations of compositions and structures of PHA's to be produced. In the field of biomedical materials, there are already considerable results about the use of such biodegradable resins. In addition, in the field of agriculture, the resins are put into practical applications including multi-files, gardening materials, extended-release pesticides, and fertilizers. Furthermore, in the field of leisure, the biodegradable resins are used for fishing lines, fishing articles, golf articles, and so on. In addition, the resins are practically used as packing materials of daily necessities such as containers of livingwares. However, considering the wide applications of the resins as plastic materials, the physical properties of the respective resins are still insufficient under present circumstances. It is important to consider improvements in physical properties of the PHA's more widely for further improving their application ranges. Therefore, further development of or searches for PHA's that contain monomer units having various structures have been indispensable.
In the field of electrophotography, furthermore, there is proposed a method using a biodegradable resin as a binder resin for realizing a toner which can be disposed without causing any environmental contamination. Japanese Patent Application Laid-Open No. H06-289644 discloses an electrophotographic toner particularly used for thermal roll fixation, which is characterized in that a binder resin contains at least a plant wax and a biodegradable resin (e.g., polyester produced by a microorganism or a natural polymeric material originated from a plant or an animal), and that the content of the plant wax in the binder resin is in the range of 5 to 50% by mass. In addition, Japanese Patent Application Laid-Open No. H08-262796 discloses an electrophotographic toner having a binder resin and a colorant, which is characterized in that the binder resin is a biodegradable resin (e.g., aliphatic polyester resin) and the colorant is a water-insoluble pigment. Furthermore, U.S. Pat. No. 5,004,664 discloses a toner containing, as its composition, poly-3-hydroxybutyric acid, poly-3-valeric acid, or a copolymer, or a blend thereof. In those technologies, the binder resins are biodegradable, so the toners can be surely decomposed in soil when the toners are embedded in the soil. However, there are some problems with respect to essential functions of the binder resin in each of the above toners. For instance, the toner is less durable, and also the binder resin is highly hygroscopic and its electrostatic property is instable. Specifically, poly-3-hydroxybutyric acid is a hard and brittle material having a melting point of 180° C., a crystallinity of 50-70%, a Young's modulus of 3.5 GPa, and an elongation at break of 5%. Therefore, such a material is inadequate to be provided as a binder resin of the toner in practice.
In addition, there is proposed a toner mainly composed of polylactate aliphatic polyester, which has biodegradability and is efficiently decomposed in alkali hydrolysis and useful for recycling the used paper. Japanese Patent Application Laid-Open No. H07-120975 proposes a method for making a lactate homopolymer into a toner. In this document, a polylactic acid prepared by a ring-opening polymerization method is provided as a representative example of the homopolymer.
In the ring-opening polymerization method, lactic acid is oligomerized by a dehydration reaction at first, and the resulting oligomer is depolymerized to make the lactic acid into a lactide (a cyclic dimer), followed by subjecting the lactide to further ring-opening polymerization. Because the method follows such a complicated process, the resulting polylactic acid is too expensive to be used as a toner resin.
In addition, the ring-opening polymerization is of a cationic type, so that the polymerization requires making a solvent to be employed anhydrous, removable of ionic species to be provided as polymerization terminators, and so on. As a result, the ring-opening polymerization leads to poor producibility, and the monomer species that can be used for the production of polyester are limited to cyclic esters. Thus, it is difficult to provide the binder resin with physical properties desired for the toner resin. In addition, it is also difficult to prepare a copolymer with any of various monomers for controlling a balance between the degradability and the physical property. In this regard, cheap and degradable polyester the physical properties of which can be easily controlled has been demanded in the art. Additionally, when a polylactic acid is directly prepared as a toner, there are problems in storage stability and offset resistance of the toner. Thus, such a toner is not yet in the actual use.
Furthermore, Japanese Patent Application Laid-Open No. H09-274335 discloses a toner for electrostatic charge image development, which is characterized by containing: a polyester resin obtained by a dehydrating polycondensation reaction of a composition that contains lactic acid and oxycarboxylic acid having three or more functional groups; and a colorant. In this document, however, the polyester resin is prepared by the dehydrating polycondensation reaction between an alcohol group in the lactic acid and a carboxylic acid group in the oxycarboxylic acid, so that the molecular weight of the resulting resin may be higher than the desired one. Thus, the higher molecular weight is expected to lead to a decrease in biodegradability of the resin. Furthermore, as is the case with Japanese Patent Application Laid-Open No. H07-120975, there are problems in storage stability and offset resistance of the toner.
Moreover, a polycaprolactone, which is a typical single polymer of hydroxycarboxylic acid, has a low melting point and a low glass transition point, and is excellent in compatibility with various kinds of resins. However, the polycaprolactone has a melting point as low as 60° C., so that the polycaprolactone cannot be suitable for a binder resin when the polycaprolactone is used alone. In addition, polylactic acid has a high glass transition point (60° C.), and also polylactic acid having a crystalline form is a thermoplastic macromolecule having a high melting point (about 180° C.). However, as described above, the polylactic acid is not yet in the actual use as a binder resin. Furthermore, the conventional toner resin made of a biodegradable polyester shows poor grindability in general. Thus, it is difficult to use such a toner resin a binder resin that accounts for 90% of a toner having a particle size of about 10 μm. Considering the practical use of toner as a binder resin, improvements in the physical properties of the toner have been strongly demanded.
Incidentally, like the conventional plastic material, the PHA described above can be used for the production of various products by melt-processing or the like. In addition, the PHA has an advantage of being completely decomposed by microorganisms in the nature. In other words, there is no possibility that the remaining PHA pollutes the natural environment as in the case with many synthetic macromolecular compounds known in the art. In addition, the PHA is excellent in biocompatibility, so that application of the PHA to a medical soft material or the like is also expected.
As described above, it has been known that the microbial produced PHA's can have various compositions and structures depending on the species of microorganisms to be used for the production, compositions of media, conditions for culturing these microorganisms, and so on. Mainly from the view point of improving the physical properties of PHA's, the studies for adjusting the compositions and structures of the PHA's have been carried out up to now.
In particular, the biosynthesis of a PHA by polymerizing a monomer unit having a comparatively simple structure, such as 3-hydroxy-n-butyric acid (hereinafter, abbreviated as “3HB”), 3-hydroxy-n-valeric acid (hereinafter, abbreviated as “3HV”), 3-hydroxy-n-hexanoic acid (hereinafter, abbreviated as “3HHx”), or 4-hydroxy-n-butyric acid (hereinafter, abbreviated as “4HB”) has been investigated, and the PHA production from various microorganisms has been reported.
Furthermore, in recent years, a PHA that is composed of a 3-hydroxyalkane acid with a medium chain length (abbreviated as “mcl”) having up to 12 carbons has been under intense study. The production of mcl-PHA using a noncyclic aliphatic hydrocarbon, octanoic acid, hexanoic acid, sodium gluconate, or the like as a carbon source has been confirmed.
By the way, the PHA's synthesized in the above example are those composed of monomer units having alkyl groups at their respective side chains. In other words, therefore, those PHA's are “usual PHA's”.
However, considering broader applications of PHA's, for example applications of PHA's to functional polymers, it is expected that the PHA where a substituent except an alkyl group introduced in the side chain thereof, i.e., “unusual PHA”, is extremely useful. Examples of such a substituent include those having aromatic rings (such as a phenyl group, a phenoxy group, and a benzoyl group), an unsaturated hydrocarbon, an ester group, an allyl group, a cyano group, a halogenated hydrocarbon, and an epoxide. Among them, in particular, the PHA's having aromatic rings have been actively investigated in the art.
Reported as the production of a PHA containing a phenyl group or a partially substituted product thereof are, for example: production of a PHA containing 3-hydroxy-5-phenylvaleric acid as a unit using 5-phenylvaleric acid as a substrate; production of a PHA containing 3-hydroxy-5-(4′-tolyl)valeric acid as a unit using 5-(4′-tolyl)valeric acid as a substrate; and production of a PHA containing 3-hydroxy-5-(2′,4′-dinitrophenyl)valeric acid and 3-hydroxy-5-(4′-nitrophenyl)valeric acid as units using 5-(2′,4′-dinitrophenyl)valeric acid as a substrate. Reported as the production of a PHA containing a phenoxy group or a partially substituted product thereof are, for example: production of a PHA copolymer of 3-hydroxy-5-phenoxyvaleric acid and 3-hydroxy-9-phenoxynonanoic acid using 11-phenoxyundecanoic acid as a substrate; production of a PHA containing a 3-hydroxy-4-phenoxybutyrate unit and a 3-hydroxy-6-phenoxyhexanoate unit from 6-phenoxyhexanoic acid; production of a PHA containing a 3-hydroxy-4-phenoxybutyrate unit, a 3-hydroxy-6-phenoxyhexanoate unit, and a 3-hydroxy-8-phenoxyoctanoate unit from 8-phenoxyoctanoic acid; and production of a PHA containing a 3-hydroxy-5-phenoxyvalerate unit and a 3-hydroxy-7-phenoxyheptanoate unit from 11-phenoxyundecanoic acid. In addition, reported are a PHA homopolymer of a 3-hydroxy-5-(monofluorophenoxy)pentanoate (3H5(MFP)P) unit or a 3-hydroxy-5-(difluorophenoxy)pentanoate (3H5(DFP)P) unit and a PHA copolymer containing at least a 3H5(MFP)P unit or a 3H5(DFP)P unit. The effect thereof is that stereoregularity and repellency are given while a high melting point and excellent processability are being kept. In addition to the fluorine-group-substituted products described above, cyano-group-substituted products and nitro-group-substituted products have also been investigated. For example, reported is production of a PHA containing 3-hydroxy-p-cyanophenoxyhexanoic acid or 3-hydroxy-p-nitrophenoxyhexanoic acid as a monomer unit using octanoic acid and p-cyanophenoxyhexanoic acid or p-nitrophenoxyhexanoic acid as substrates. The PHA's described in those reports differ from a general PHA having an alkyl group in a side chain, and each of the PHA's has an aromatic ring at the side chain, which is beneficial in obtaining a polymer having physical properties derived from the aromatic ring. In addition, as an unusual PHA having a cyclohexyl group, for example, production of the PHA from cyclohexylbutyric acid or cyclohexylvaleric acid is reported.
As described above, the microbial produced PHA's may have various kinds of compositions and structures by changing species of the microorganisms to be used for the production, compositions of media, conditions for culturing these microorganisms, and so on. However, considering their applications to plastic materials, the physical properties of the PHA's are still insufficient. For extending the application scope of a microbial produced PHA, it is important to more broadly consider improvements in its physical properties. Therefore, it is indispensable to further develop and search PHA's that contain monomer units having various structures respectively, the production method thereof, and microorganisms that can effectively produce desired PHA's.
On the other hand, a PHA of a type having a side chain in which a substituent is introduced as described above (i.e., unusual PHA) can be expected to be deployed as a “functional polymer” having extremely useful functions and characteristics originating from the characteristics or the like of the introduced substituent by selecting the introduced substituent depending on the desired characteristics or the like. Therefore, it is also an important objective to develop and search such an excellent PHA with compatibility between such functionality and biodegradability, a production method thereof, and a microorganism capable of effectively producing the desired PHA.